This application is based on patent application No. 10-313703 filed in Japan, the contents of which are hereby incorporated by references.
1. Field of the Invention
This invention relates to a rubber composition suitable for injection molding, more particularly to a rubber composition suitable for a golf ball, a golf ball provided with a vulcanized rubber composition layer and having required performance characteristics, and a method for producing such a golf ball at a high productivity.
2. Description of the Prior Art
A core and other parts of a golf ball are generally made by compressively vulcanizing and molding a rubber composition comprising: a diene rubber; a co-crosslinking agent, such as acrylic acid, methacrylic acid, or its metal salt; and an initiator for initiating graft polymerization of the diene rubber and the co-crosslinking agent, such as organic peroxides. In the compressive vulcanization molding, it has been required to accurately measure the weight of rubber composition to be fed into a mold for the reason that a variation in the feeding amount of rubber composition causes a variation in the weight of products. However, this strict weight measurement has lowered the production efficiency. Further, the rubber composition is vulcanized by heat transmitted through the wall of the mold which is heated by a specified vulcanization temperature before vulcanizing the rubber composition. This heating process requires a long time. In particular, vulcanization of a golf ball core, which has a great thickness, takes a considerably long time. This makes it difficult to reduce the time for vulcanization and molding, consequently hindering the improvement of productivity.
In order to solve the foregoing problems, in recent years, it has been discussed to utilize injection molding for production of golf ball core. The injection molding has been expected to decrease the molding cycle time and avoid the product weight variation due to variations in the weighing in the compressive vulcanization molding of golf ball core. Conventionally, most of injection molding machines are designed for molding of thermoplastic resin. However, it can be seen that such injection molding machines are utilizable to molding rubber compositions and thermosetting resins by modifying the construction of an injection molding machine and the operation temperature of a cylinder of the machine and the mold so as to be suitable for production of rubber and thermosetting resin. In vulcanization molding of rubber composition, especially, injection molding is advantageous in the aspect of cost reduction because of the fact that heat occurs owing to internal friction in the rubber composition moving in a cylinder passage extending to the mold, and it is not thus necessary to heat and keep the cylinder at a high temperature. Further, since the rubber composition is imparted with sufficient heat caused by the internal friction to have a temperature near a vulcanization temperature when entering the mold. Therefore, the mold can be heated to a specified vulcanizaition temperature in a shorter time than in the compressive vulcanization molding.
In the case of injection molding a rubber composition, the cylinder of the injecting machine is heated to soften the rubber composition. However, this heating is liable to cause the following problem. Specifically, crosslinking of the rubber composition starts due to the heated cylinder and the heat caused by internal friction in the rubber composition. After the crosslinking starting, the viscosity of the rubber composition increases and the fluidity of the rubber composition decreases. The decreased fluidity increases the heat of internal friction, consequently accelerating vulcanization. Finally, the rubber composition is entirely vulcanized, which thus making a desired molding impossible. Especially, a rubber composition for golf ball, which includes an organic peroxide as crosslinking agent (or vulcanization initiator), has a greater initial vulcanization rate than a rubber composition including sulfur or the like as crosslinking agent. Accordingly, a keen control must be performed to prevent vulcanization from being initiated in the cylinder passage.
To prevent scorching (i.e., premature vulcanization) or maintain a proper fluidity, it can be proposed to keep the temperature of the cylinder lower, or to increase the area of a gate (or inflow opening) of the mold. However, these manners make the vulcanization and molding operation longer, and thus lessen the advantageous effect of the injection molding, that is, raising the productivity. Further, even if a rubber composition having been subject to scorching is fed in the mold by increasing the area of the gate or increasing the transfer force of a screw in the cylinder, and successfully vulcanized in the mold to form a desired product, the thus molded product is likely to have a poor elasticity and resilience, and an anisotropies due to remaining internal stress. Accordingly, it can be seen that this manner is improper for production of rubber products requiring resilience and isotropy, such as golf ball.
Further, it can be proposed to add a plasticizer or oil to the rubber composition to increase its fluidity and prevent heat of internal friction. However, there is the problem that some of the added plasticizer or oil blooms on a surface of the molded rubber composition. In the case of the molded rubber composition being a core of a two-piece golf ball, there is a likelihood that plasticizer or oil on a surface of the core migrate to a cover enclosing the core, thereby swelling the cover. The swell of the cover degrades the strength and rigidity of the cover, and then lowers the durability and resilience of the golf ball.
It is an object of the present invention to provide a rubber composition for injection molding, a golf ball, and a golf ball producing method which have overcome the problems residing in the prior art.
It is another object of the present invention to provide a rubber composition for injection molding and a golf ball producing method which make it possible to assure a proper fluidity and temperature of a rubber composition in an injection cylinder to prevent scorching from occurring in the composition, thereby producing a vulcanized and molded product having required performance characteristics.
According to an aspect of the present invention, a rubber composition for injection molding comprises a diene rubber; a crosslinking agent; an xcex1, xcex2-unsaturated carboxylic acid and/or a metal salt thereof; and xcex1, xcex2-unsaturated carboxylic alkylester. The ratio of the total content of the xcex1, xcex2-unsaturated carboxylic acid and the metal salt to the content of the alkylester, namely, [(carboxylic acid+metal salt thereof)/ester], ranges from 1.0 to 12.
According to another aspect of the invention, a golf ball comprises a rubber part made from the above-mentioned inventive rubber composition.
According to still another aspect of the invention, a method for producing a golf ball, comprises injecting the above-mentioned inventive rubber composition into a mold having a gate part area of 0.4 mm2 to 20 mm2; and vulcanizing the rubber composition in the mold.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.